Failure of local Mie theory: optical spectra of colloidal aggregates

Fully retarded calculations of the optical properties of aggregates of meta llic nanoparticles have been performed. Within a local theory, the optical spectra of particles in contact with each other are dominated by a series o f resonances below the Frohlich resonance of isolated spheres. The resonanc es are shown to be related to surface plasmon polaritons of extremely short wavelength, which are localised at the points of contact of the particles, and are connected with strong field enhancements in the vicinity of the co ntact point. Standard local Mie theory of the optical properties of interac ting spheres does not describe this excitation correctly at finite cut-off multipole orders less than or equal to 30. Intensity distributions and spec tra were therefore calculated by means of the semianalytical multiple multi pole technique. For small particles, however, a nonlocal refinement of aggr egate Mie theory predicts, that the localised surface excitations are stron gly damped by the excitation of volume plasmons at the surface. Local Mie t heory should therefore not be applied to colloidal aggregates of small meta llic particles in contact with each other, The localised surface plasmon po laritons are important, on the Ether hand, for aggregates of somewhat large r particles, even if nonlocality is taken into account. Employing aggregate Mie theory with a nonlocal dielectric function, we show in addition, that, even for relatively small particles, retardation effects and high-order mu ltipoles must be taken into account. (C) 2001 Published by Elsevier Science B.V.